[runtimes] Improve the documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS

[ldionne]

This clarifies how that option is meant to be used to avoid confusion. As a drive-by, also fix an incorrect usage in the recently-added GPU caches.

[llvmbot]

@llvm/pr-subscribers-libunwind
@llvm/pr-subscribers-backend-amdgpu
@llvm/pr-subscribers-libcxxabi

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

This clarifies how that option is meant to be used to avoid confusion. As a drive-by, also fix an incorrect usage in the recently-added GPU caches.

---

Full diff: https://github.com/llvm/llvm-project/pull/112733.diff

5 Files Affected:

• (modified) libcxx/CMakeLists.txt (+3-1)
• (modified) libcxx/cmake/caches/AMDGPU.cmake (+2-2)
• (modified) libcxx/docs/VendorDocumentation.rst (+4-8)
• (modified) libcxxabi/CMakeLists.txt (+1-2)
• (modified) libunwind/CMakeLists.txt (+1-2)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 75c926f5432aea..723863ddfd54d4 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -459,7 +459,9 @@ set(LIBCXX_COMPILE_FLAGS "")
 set(LIBCXX_LINK_FLAGS "")
 set(LIBCXX_LIBRARIES "")
 set(LIBCXX_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+    "Additional compile flags to use when building libc++. This should be a CMake ;-delimited list of individual
+     compiler options to use. For options that must be passed as-is to the compiler without deduplication (e.g.
+     `-Xclang -foo` option groups), consider using `SHELL:` (https://cmake.org/cmake/help/latest/command/add_compile_options.html#option-de-duplication).")
 set(LIBCXX_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libc++ is linked to which can be provided in cache")
 
diff --git a/libcxx/cmake/caches/AMDGPU.cmake b/libcxx/cmake/caches/AMDGPU.cmake
index c7d6afc854a54c..1a6bfd85a50be3 100644
--- a/libcxx/cmake/caches/AMDGPU.cmake
+++ b/libcxx/cmake/caches/AMDGPU.cmake
@@ -29,7 +29,7 @@ set(LIBCXXABI_USE_LLVM_UNWINDER OFF CACHE BOOL "")
 
 # Necessary compile flags for AMDGPU.
 set(LIBCXX_ADDITIONAL_COMPILE_FLAGS
-    "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
+    "-nogpulib;-flto;-fconvergent-functions;SHELL:-Xclang -mcode-object-version=none" CACHE STRING "")
 set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS
-    "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
+    "-nogpulib;-flto;-fconvergent-functions;SHELL:-Xclang -mcode-object-version=none" CACHE STRING "")
 set(CMAKE_REQUIRED_FLAGS "-nogpulib" CACHE STRING "")
diff --git a/libcxx/docs/VendorDocumentation.rst b/libcxx/docs/VendorDocumentation.rst
index 3a3d1cdb1ea7ff..3795381264c93b 100644
--- a/libcxx/docs/VendorDocumentation.rst
+++ b/libcxx/docs/VendorDocumentation.rst
@@ -213,11 +213,13 @@ General purpose options
 
   Output name for the shared libc++ runtime library.
 
-.. option:: LIBCXX_ADDITIONAL_COMPILE_FLAGS:STRING
+.. option:: {LIBCXX,LIBCXXABI,LIBUNWIND}_ADDITIONAL_COMPILE_FLAGS:STRING
 
   **Default**: ``""``
 
-  Additional Compile only flags which can be provided in cache.
+  Additional compile flags to use when building the runtimes. This should be a CMake ``;``-delimited list of individual
+  compiler options to use. For options that must be passed as-is to the compiler without deduplication (e.g.
+  ``-Xclang -foo`` option groups), consider using ``SHELL:`` as `documented here <https://cmake.org/cmake/help/latest/command/add_compile_options.html#option-de-duplication>`_.
 
 .. option:: LIBCXX_ADDITIONAL_LIBRARIES:STRING
 
@@ -346,12 +348,6 @@ The following options allow building libc++ for a different ABI version.
   Build and use the LLVM unwinder. Note: This option can only be used when
   libc++abi is the C++ ABI library used.
 
-.. option:: LIBCXXABI_ADDITIONAL_COMPILE_FLAGS:STRING
-
-  **Default**: ``""``
-
-  Additional Compile only flags which can be provided in cache.
-
 .. option:: LIBCXXABI_ADDITIONAL_LIBRARIES:STRING
 
   **Default**: ``""``
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index ac1ee69d5f11c9..da0e8b286cddc1 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -222,8 +222,7 @@ set(LIBCXXABI_CXX_FLAGS "")
 set(LIBCXXABI_COMPILE_FLAGS "")
 set(LIBCXXABI_LINK_FLAGS "")
 set(LIBCXXABI_LIBRARIES "")
-set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING "See documentation LIBCXX_ADDITIONAL_COMPILE_FLAGS")
 set(LIBCXXABI_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libc++abi is linked to which can be provided in cache")
 
diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index b911f482fc26b2..ea06dc8a67b949 100644
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -162,8 +162,7 @@ set(LIBUNWIND_C_FLAGS "")
 set(LIBUNWIND_CXX_FLAGS "")
 set(LIBUNWIND_COMPILE_FLAGS "")
 set(LIBUNWIND_LINK_FLAGS "")
-set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING "See documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS")
 set(LIBUNWIND_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libunwind is linked to which can be provided in cache")